1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to solid golf balls having a core, a mid layer and a cover.
2. Description of the Related Art
In recent years, three-piece golf balls were developed in attempts to accomplish a feel at impact that is comparative to wound balls, and have been supplied on the market. U.S. Pat. No. 6,299,551 and U.S. Pat. No. 6,213,896 disclose three-piece golf balls having a mid layer that includes a thermoplastic elastomer. U.S. 2002/173380 and U.S. 2003/040378 disclose three-piece golf balls having a mid layer that includes an ionomer resin and a thermoplastic elastomer.
Golf balls have numerous dimples on the surface thereof. In general, golf balls have single radius dimples having a cross-sectional shape with single curvature radius, or double radius dimples having a cross-sectional shape with two curvature radii. The dimples disrupt the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separating points of the air from the golf ball shift backwards leading to the reduction of a drag. The turbulent flow separation promotes the differentia between the separating point on the upper side and the separating point on the lower side of the golf ball, which result from the backspin, thereby enhancing the lift force that acts upon the golf ball. Such a role of the dimples is referred to as a “dimple effect”. Excellent dimples disturb the air flow more efficiently.
A variety of proposals with respect to the shape of the dimples in attempts to improve flight performances have been made. U.S. Pat. No. 5,338,039 discloses dimples having a shape with the gradient of a slope disposed in the vicinity of the edge being greater than that of a slope at the bottom part. U.S. Pat. No. 5,735,757 discloses dimples having a cross-sectional shape given by double radius. U.S. 2004/142771 discloses dimples having a great ratio of the curvature radius of the bottom part to the curvature radius in the vicinity of the edge. U.S. 2003/153403 discloses a golf ball having a mid layer including an ionomer resin and a thermoplastic elastomer, and having dimples with a great contour length.
According to the golf balls disclosed in U.S. Pat. No. 6,299,551 and U.S. Pat. No. 6,213,896, resilience performance of the mid layer is insufficient. The flight performance correlates to the resilience performance. Therefore, these golf balls are inferior in the flight performance. According to these golf balls, the flight performance is sacrificed for the sake of the feel at impact.
According to the golf ball disclosed in U.S. 2002/173380, spin at shots with a driver tends to be excessive resulting from high hardness of the mid layer. Excessive spin contributes to insufficient flight distance. The mid layer with high hardness also adversely affects the feel at impact. According to the golf ball disclosed in U.S. 2003/040378, sufficient flight distance is not attained also due to excessive spin.
Top concern to golf players for golf balls is the flight distance. In light of the flight distance, there remains room for improvement of the dimple. When a golf ball is hit with a short iron, the surface thereof may be scuffed. In case of golf balls having double radius dimples, in particular, margin of the dimple is liable to be scuffed resulting from concentration of the stress. There also remains room for improvement of the dimple in respect of the scuff resistance.
An object of the present invention is to provide a golf ball that is excellent in the feel at impact, flight performance and scuff resistance.